U.S. patent application number 10/265632 was filed with the patent office on 2003-06-19 for cross current compensation control system for a power system.
This patent application is currently assigned to Mitsubishi Denki Kabushiki Kaisha. Invention is credited to Ashizawa, Tomoo, Matsumoto, Tadashi, Nomura, Toshio, Tange, Seiji.
Application Number | 20030111907 10/265632 |
Document ID | / |
Family ID | 19187368 |
Filed Date | 2003-06-19 |
United States Patent
Application |
20030111907 |
Kind Code |
A1 |
Tange, Seiji ; et
al. |
June 19, 2003 |
Cross current compensation control system for a power system
Abstract
A cross current compensation control system includes a cross
current detection compensator having an input terminal to which a
cross connection line that cross-connects the secondary sides of
auxiliary CTs are connected, and an output terminal to which cross
current compensation lines that connect the secondary sides of a
plurality of CTs in series are connected, in which the cross
current detection compensator supplies a compensation current to
the cross current compensation line so as to cancel the current
component corresponding to the cross current that appears at the
secondary sides of the CTs when detecting the cross current that
circulates within the bus bar, the distributions and the
switch.
Inventors: |
Tange, Seiji; (Tokyo,
JP) ; Matsumoto, Tadashi; (Tokyo, JP) ;
Ashizawa, Tomoo; (Tokyo, JP) ; Nomura, Toshio;
(Tokyo, JP) |
Correspondence
Address: |
LEYDIG VOIT & MAYER, LTD
700 THIRTEENTH ST. NW
SUITE 300
WASHINGTON
DC
20005-3960
US
|
Assignee: |
Mitsubishi Denki Kabushiki
Kaisha
Tokyo
JP
|
Family ID: |
19187368 |
Appl. No.: |
10/265632 |
Filed: |
October 8, 2002 |
Current U.S.
Class: |
307/17 |
Current CPC
Class: |
H02J 3/48 20130101; H02J
3/46 20130101 |
Class at
Publication: |
307/17 |
International
Class: |
H02J 003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 14, 2001 |
JP |
2001-381682 |
Claims
What is claimed is:
1. A cross current compensation control system for a power system,
comprising: first and second current transformers disposed at the
respective sending ends of first and second power systems connected
to the same bus bar; a plurality of third current transformers
disposed in each of sections of the first and second power systems;
and a cross current detection compensator having an input terminal
and an output terminal in which a cross connection line that
cross-connects a secondary side of the first and second current
transformers is connected to the input terminal and a cross current
compensation line that connects the secondary sides of the
plurality of third current transformers in series is connected to
the output terminal, wherein the cross current detection
compensator supplies a compensation current to the cross current
compensation line so as to cancel a current component corresponding
to the cross current that appears at the secondary sides of the
plurality of the third current transformers when the cross current
detection compensator detects a cross current that circulates in a
switch that connects the bus bar, the first and second power
systems and the first and second power systems.
2. A cross current compensation control system for a power system
according to claim 1, further comprising a contact that is
connected between the output terminals of the cross current
detection compensator and closed when the switch that connects the
first and second power systems is opened.
3. A cross current compensation control system for a power system
according to claim 1, further comprising: a first contact that is
connected between the output terminals of the cross current
detection compensator and closed when the switch that connects the
first and second power systems is opened; and a second contact that
is connected between the input terminals of the cross current
detection compensator, and closed when the switch that connects the
first and second power systems is opened.
4. A cross current compensation control system for a power system,
comprising: first and second current transformers disposed at the
respective secondary sides of first and second power transformers
connected to the same power supply; a cross current detection
compensator connected to a cross connection line that
cross-connects the secondary sides of the first and second current
transformers; a first voltage adjusting device connected to a first
bus bar connected to the secondary sides of the cross current
detection compensator and the first power transformer; and a second
voltage adjusting device connected to a second bus bar connected to
the secondary sides of the cross current detection compensator and
the second power transformer, wherein: the cross current detection
compensator outputs a compensation voltage when detecting a cross
current that circulates within the first and second power
transformers, the first and second bus bars and a switch that
connects the first and second bus bars; the first voltage adjusting
device controls the voltage adjustment tap at the secondary side of
the first power transformer on the basis of the voltage of the
first bus bar and the compensation voltage; and the second voltage
adjusting device controls the voltage adjustment tap at the
secondary side of the second power transformer on the basis of the
voltage of the second bus bar and the compensation voltage.
5. A cross current compensation control system for a power system
according to claim 4, wherein the cross current detection
compensator comprises a contact that is connected between the input
terminals of the cross current detection compensator and closed
when the switch that connects the first and second bus bars is
opened.
6. A cross current compensation control system for a power system
according to claim 4, wherein the cross current detection
compensator comprises a contact that is connected between the
output terminals of the cross current detection compensator and
closed when the switch that connects the first and second bus bars
is opened.
7. A cross current compensation control system for a power system
according to claim 4, wherein the cross current detection
compensator comprises: a first contact that is connected between
the input terminals of the cross current detection compensator and
closed when the switch that connects the first and second bus bars
is opened; and a second contact that is connected between the
output terminals of the cross current detection compensator and
closed when the switch that connects the first and second bus bars
is opened.
8. A cross current compensation control system for a power system,
comprising: a first current transformer disposed at the secondary
side of a first power transformer connected to a first power
supply; a second current transformer disposed at the secondary side
of a second power transformer connected to a second power supply; a
cross current detection compensator connected to a cross connection
line that cross-connects the secondary sides of the first and
second current transformers; a first voltage adjusting device
connected to a first bus bar connected to the secondary sides of
the cross current detection compensator and the first power
transformer; and a second voltage adjusting device connected to a
second bus bar connected to the secondary sides of the cross
current detection compensator and the second power transformer,
wherein: the cross current detection compensator outputs a
compensation voltage when detecting a cross current that circulates
within the first and second power transformers, the first and
second bus bars and a switch that connects the first and second bus
bars; the first voltage adjusting device controls the voltage
adjustment tap at the secondary side of the first power transformer
on the basis of the voltage of the first bus bar and the
compensation voltage; and wherein the second voltage adjusting
device controls the voltage adjustment tap at the secondary side of
the second power transformer on the basis of the voltage of the
second bus bar and the compensation voltage.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a cross current
compensation control system for a power system which compensates a
cross current during the parallel operation of a power transmission
line, a power transformer, a device such as a distribution and
equipment in the power system. For convenience of description, the
operation of the distribution system will be mainly described.
[0003] 2. Description of the Related Art
[0004] A conventional parallel operation of the distribution system
will be described with reference to the accompanying drawings. FIG.
15 is a circuit diagram showing a conventional distribution
parallel operation of the distribution system.
[0005] Referring to FIG. 15, reference numeral 1 denotes a power
supply; 2 is a primary side circuit breaker of a power transformer
(Tr); 3 is a primary side winding of the power transformer; 4 is a
secondary side winding of the power transformer; 5 is a secondary
side circuit breaker of the power transformer; and 6 is a bus
bar.
[0006] Also, in the figure, reference numeral 10 denotes a
distribution; 10-1 is a circuit breaker of the distribution 10;
11-1, 11-2, 11-3, 11-4 and 11-5 are sections in the distribution
10; and 10-2, 10-3, 10-4 and 10-5 are section switches or circuit
breakers (hereinafter referred to as "switches") of the
distribution 10.
[0007] Also, in the figure, reference numeral 20 denotes a
distribution; 20-1 is a circuit breaker of the distribution 20;
21-1, 21-2, 21-3, 21-4 and 21-5 are sections in the distribution
20; and 20-2, 20-3, 20-4 and 20-5 are section switches or circuit
breakers (hereinafter referred to as "switches") of the
distribution 20.
[0008] In addition, in the figure, reference numeral 12 denotes a
CT (current transformer) of the distribution 10; 13 is a device
that monitors and protects the distribution 10 (hereinafter
referred to as "protection relay" for convenience of description);
22 is a CT (current transformer) of the distribution 20; 23 a
device that monitors and protects the distribution 20 (hereinafter
referred to as "protection relay" for convenience of description);
and 30 is a distribution connection switch or circuit breaker
(hereinafter referred to as "switch"), between distribution 10 and
20.
[0009] In FIG. 15, the CT 12 of the distribution 10 and the CT 22
of the distribution 20 derive currents in the respective phases by
using three CTs in a direct ground system of the distribution
system, and an over-current relay (one of the protection relays) is
located at their secondary sides, respectively, to protect against
the over-load and short-circuiting fault of the distributions 10
and 20. In addition, a ground over-current relay (one of the
protection relays) is disposed in a residual circuit at the
secondary side of each of the CTs 12 and 22, respectively, to
protect against the ground fault. Also, a CT and a protection relay
are disposed in each of the section switches 10-2, 10-3, . . . ,
20-2, 20-3, . . . and 30, as in the sending ends of the
distributions 10 and 20, to monitor and protect the respective
sections 11-2, 11-3, . . . and 21-2, 21-3, . . . .
[0010] Also, in FIG. 15, the CT 12 of the distribution 10 and the
CT 22 of the distribution 20 derive currents in the respective
phases by using two CTs in a high resistance ground system
(including a GPT ground system) of the distribution system, and an
over-current relay is located at their secondary sides,
respectively, to protect against the over-load and short-circuiting
fault of the distributions 10 and 20. Also, the CT 12 and the CT 22
derive a zero-phase current and a zero-phase voltage by using a ZCT
and a GPT, and generally connect a ground direction relay to
conduct a ground protection. Also, a CT and a protection relay
which are similar to those at the sending ends of the distributions
10 and 20 are disposed in each of the section switches 10-2, 10-3,
. . . , 20-2, 20-3, . . . and 30, to monitor and protect the
respective sections 11-2 11-3, . . . and 21-2, 21-3, . . . .
[0011] Subsequently, the operation of the conventional distribution
system will be described with reference to the accompanying
drawings.
[0012] It has been known that a cross current 50 of a zero-phase
flows when the switch 30 is closed by the operation of the
distribution in FIG. 15. This is because the cross current 50 of
the zero-phase occurs because impedances in the respective phases
in a closed loop have slight differences even if supply voltages
are identical.
[0013] The occurrence of the cross current will be described now.
FIG. 16 shows a principle diagram of the generation of across
current. In the figure, references Ea, Eb and Ec denote supply
voltages; Za1, Zb1 and Zc1 are impedances at the distribution 10
side; and Za2, Zb2 and Zc2 are impedances at the distribution 20
side.
[0014] In this example, an actual distribution system is that when
the switch 30 is closed, the above impedances are not completely
identical with each other strictly, but very slightly in an
unbalanced state in the respective phases. The unbalance is caused
by, for example, a slight difference of the lengths of electric
wires in the respective phases and a light difference of the
contact resistances at nodes of the respective electric wires.
Also, the unbalance is caused by a difference of a voltage drop in
each of the phases due to the load unbalance of each of the phases,
etc. There has been known a fact that an unbalanced voltage
develops within a loop even if the supply voltage is of a synthetic
three-phase voltage.
[0015] That is, in FIG. 16, the voltages in the respective phases
when the switch 30 is opened are represented by a table of FIG. 17.
In the table, the phase voltages at the distribution 10 side are
Ea1, Eb1 and Ec1, and the phase voltage at the distribution 20 side
are Ea2, Eb2 and Ec2, and differential voltages of the respective
phases are represented by .DELTA.Ea, .DELTA.Eb and .DELTA.Ec. It is
general that those differential voltages .DELTA.Ea, .DELTA.Eb and
.DELTA.Ec cannot become the synthetic three-phase voltages due to a
difference of the impedances of the respective phases and a
difference of the load currents of the respective phases, as
described above.
[0016] Subsequently, the magnitude of the cross current will be
described. A positive-phase voltage, a negative-phase voltage and a
zero-phase voltage called in a method of symmetric coordinates
exist in an asymmetric three-phase voltage, and it is apparent that
the magnitude of the cross current is determined by those voltages
and a positive-phase impedance, a negative-phase impedance and a
zero-phase impedance of the closed circuit. Although being
dependent on the system, in this case, a fact that the magnitude of
the cross current of the zero phase becomes about 1 to 10 A has
been observed in the actual system.
[0017] The primary rating of the CT as generally used is 400 A or
600 A. In this example, the positive-phase and negative-phase
currents are relatively small in comparison with the detection
level of the over-load and the short-circuiting protection relay
(about 5 A to 6 A of the CT secondary rating or more), and the
zero-phase current becomes a value close to the detection level
(about 0.1 A to 0.5 A) of the ground protection relay of the direct
ground system. It is readily presumable that the current exceeds
the detection level of the ground protection relay (about 0.2 to
0.5 A at the ZCT primary side).
[0018] The parallel operation of another conventional distribution
system will be described with reference to the accompanying
drawings. FIG. 18 is a circuit diagram showing a transformer in the
same power supply in the conventional distribution system and the
parallel operation of the distribution system. Also, FIG. 19 is a
circuit diagram showing a transformer in the same power supply in
the conventional distribution system and the parallel operation
through the distribution system.
[0019] FIGS. 18 and 19 are diagrams showing the parallel operation
of the power transformer and the parallel operation of the
distribution. In the figures, reference 2-1 denotes a primary side
circuit breaker; 3-1 is a primary side winding of the power
transformer; 4-1 is a secondary side winding of the power
transformer; 5-1 is a secondary side circuit breaker; 6-1 is a bus
bar; and 7 is a bus bar connection circuit breaker. Other symbols
are identical with those in FIG. 15.
[0020] The cross current will be described. In FIG. 18, when the
switch 30 of the distributions 10 and 20 is closed, the cross
current 50 occurs as described above. When the bus bar connection
circuit breaker 7 is closed, the cross current 50-1 occurs. The
occurrence causes of the cross current 50-1 are generally the
impedance difference described above with reference to FIG. 15 as
well as a voltage adjustment tap attached to the power transformer.
The difference of the voltage adjustment tap position, and the
impedance difference of the power transformer greatly operate as a
source of generating the cross current. In this case, there has
been well known a fact that all of the cross currents of the
positive phase, the negative phase and the zero phase may lead to a
problem.
[0021] Subsequently, the magnitude of the cross current will be
described. In FIG. 18, the cross current 50 in the case where the
bus bar connection circuit breaker 7 and the switch 30 are closed
is identical with that in case of FIG. 15, and therefore its
description will be omitted. A crosscurrent 50-1 will be
described.
[0022] The short-circuiting % impedance (Z) of the power
transformer is about 5 to 10%, and a voltage of one tap of the
voltage adjustment tap is about 1 to 2%. In this example, assuming
that %Z is 7.5% in each of the power transformer in both of two
power transformers, a one-tap voltage is 1.25% and the shift is two
taps, the subsequent values are obtained through rough
calculation.
[0023] Differential voltage .DELTA.V=1.25%*2=2.5%
[0024] Closed loop Z=7.5%*2=15%
[0025] Cross current I=.DELTA.V/Z=16.7%
[0026] That is, in the power transformer where the secondary rating
is 10 MVA, 6.6 kV and 875 A, the magnitude of the cross current in
this case becomes 875 A*0.167=146 A and thus becomes a very large
value.
[0027] Subsequently, a case in which the bus bar connection circuit
breaker 7 is opened and the switch 30 is closed in FIG. 19 will be
described. In this case, the cross current becomes in a state where
the cross current described in FIG. 15 and the cross current in the
above parallel power transformer are superimposed on each other.
The cross current of the zero-phase of the high-voltage
distribution becomes much-larger as compared with that in case of
FIG. 15 and may reach several tens of A to about 100 A depending on
the circumstances.
[0028] The parallel operation of still another conventional
distribution system will be described with reference to the
accompanying drawings. FIG. 20 is a circuit diagram showing a
transformer in a different power supply system of the conventional
distribution system and the parallel operation of the distribution
system. Also, FIG. 21 is a circuit diagram for explanation of a
cross current that occurs during the parallel operation in the
different power supply in the conventional distribution system.
[0029] FIG. 20 shows a case of the parallel operation of a two
power supply system. In FIG. 20, reference 1-1 denotes another
power supply. The same references denote identical parts in FIG.
18.
[0030] The cross current will be described. Referring to FIG. 20,
the cross current in the case where the bus bar connection circuit
breaker 7 is opened and the switch 30 is closed will be described.
The cross current 50 that occurs in FIG. 20 is added with the
magnitude of the voltages of two power supplies and the voltage
phase in addition to the case of FIG. 18. In general, the parallel
of the two power supplies are permitted only when the power supply
1 and the power supply 1-1 are identical with each other in its
upstream system, and the system operation is made. It is general
that the parallel in the completely different systems is not
implemented.
[0031] Subsequently, the magnitude of the cross current will be
described. FIG. 21 is a diagram showing a power supply and an
impedance corresponding to FIG. 20. As described above, the
voltages, the impedances and the voltage phases in two power
supplies determine the magnitude of the cross current. The voltages
Ea-1, Eb-1, Ec-1 and Ea-2, Eb-2, Ec-2 shown in the figure are
applied from the same bus bar of the upstream system or a higher
upstream system, and in any case, a difference in the magnitude and
phase exists between those voltages. The magnitude of the cross
current is determined in accordance with the voltage, the phase
difference and the magnitude of the impedance within the closed
loop.
[0032] The parallel operation of the conventional distribution
system shown in FIG. 15 suffers from such a problem in that the
existing ground protection relay is adversely affected by the cross
current, in particular, the zero-phase current. That is, a purpose
of the protection relay may be lost such that the ground fault
current to be operated is offset into malfunction or malfunction is
made at a time where operation should not be conducted, in
accordance with the direction of the cross current of the
zero-phase. Also, an ohm loss may occur due to the positive-phase
and negative-phase cross currents or an excessive power (an active
component, a reactive component) is measured in the measured
value.
[0033] Also, in the parallel operation of the conventional
distribution system shown in FIG. 18, there is a case in which the
cross currents may exceed the rated current of the power
transformer because the cross currents of the positive phase and
the negative phase between the power transformers become very large
values in addition to the case shown in FIG. 15, which influences
the various protection relays and also influences the measured
value. In particular, in the case of conducting the parallel
operation only at the distribution side, it can be readily presumed
that the function of section monitoring terminals located in the
distribution and the sections is remarkably impeded.
[0034] In addition, the parallel operation of the conventional
distribution system shown in FIG. 20 suffers from the same problem
as that in FIG. 18 and has the possibility that a severe problem
occurs.
[0035] The parallel operation technique of the conventional power
system suffers from the above problems and resulting in that:
[0036] 1) the existing protection relay cannot achieve a desired
object, in particular the ground relay suffers from a problem;
[0037] 2) a useless value is contained in the existing measured
value;
[0038] 3) a useless loss occurs in the power system;
[0039] 4) an overload is induced; and
[0040] 5) a power device is damaged.
SUMMARY OF THE INVENTION
[0041] The present invention has been made in order to solve the
above-mentioned problem with the related art. Therefore an object
of the present invention is to provide a cross current compensation
control system for a power system which is capable of compensating
the cross current.
[0042] In order to achieve the above object, according to the
present invention, there is provided a cross current compensation
control system for a power system, including first and second
current transformers disposed at the respective sending ends of
first and second power systems connected to the same bus bar, a
plurality of third current transformers disposed in each of
sections of the first and second power systems, and a cross current
detection compensator having an input terminal and an output
terminal in which a cross connection line that cross-connects a
secondary side of the first and second current transformers is
connected to the input terminal and a cross current compensation
line that connects the secondary sides of the plurality of the
third current transformers in series is connected to the output
terminal.
[0043] The cross current detection compensator supplies a
compensation current to the cross current compensation line so as
to cancel a current component corresponding to the cross current
that appears at the secondary sides of the plurality of third
current transformers when the cross current detection compensator
detects a cross current that circulates within the bus bar, the
first and second power systems and a switch that connects the first
and second power systems.
[0044] According to this structure, the zero-phase input amounts at
the respective terminals can be compensated, the respective
terminals can improve the reliability of the protection function,
in particular, the reliability with respect to the ground
protection. Also, it is possible to compensate the cross current of
the sending end to the protection relay as in case of the terminal
side, and the reliability of the sending end, in particular, the
ground protection function can be improved.
[0045] Also, according to the present invention, there is provided
a cross current compensation control system for a power system
including first and second current transformers disposed at the
respective secondary sides of first and second power transformers
connected to the same power supply, a cross current detection
compensator connected to a cross connection line that
cross-connects the secondary sides of the first and second current
transformers, a first voltage adjusting device connected to a first
bus bar connected to the secondary sides of the cross current
detection compensator and the first power transformer, and a second
voltage adjusting device connected to a second bus bar connected to
the secondary sides of the cross current detection compensator and
the second power transformer.
[0046] The cross current detection compensator outputs a
compensation voltage when detecting a cross current that circulates
within the first and second power transformers, the first and
second bus bars and a switch that connects the first and second bus
bars, in which the first voltage adjusting device controls the
voltage adjustment tap at the secondary side of the first power
transformer on the basis of the voltage of the first bus bar and
the compensation voltage, and in which the second voltage adjusting
device controls the voltage adjustment tap at the secondary side of
the second power transformer on the basis of the voltage of the
second bus bar and the compensation voltage.
[0047] According to this structure, the cross current of the power
transformer that is driven in parallel can be minimized and the
power loss due to the useless cross current can be suppressed.
[0048] Further, according to the present invention, there is
provided a cross current compensation control system for a power
system including a first current transformer disposed at the
secondary side of a first power transformer connected to a first
power supply, a second current transformer disposed at the
secondary side of a second power transformer connected to a second
power supply, a cross current detection compensator connected to a
cross connection line that cross-connects the secondary sides of
the first and second current transformers, a first voltage
adjusting device connected to a first bus bar connected to the
secondary sides of the cross current detection compensator and the
first power transformer, and a second voltage adjusting device
connected to a second bus bar connected to the secondary sides of
the cross current detection compensator and the second power
transformer.
[0049] The cross current detection compensator outputs a
compensation voltage when detecting a cross current that circulates
within the first and second power transformers, the first and
second bus bars and a switch that connects the first and second bus
bars, in which the first voltage adjusting device controls the
voltage adjustment tap at the secondary side of the first power
transformer on the basis of the voltage of the first bus bar and
the compensation voltage, and in which the second voltage adjusting
device controls the voltage adjustment tap at the secondary side of
the second power transformer on the basis of the voltage of the
second bus bar and the compensation voltage.
[0050] According to this structure, the cross current in the
transformers and the distribution of the different system parallel
operation can be minimized.
BRIEF DESCRIPTION OF THE DRAWINGS
[0051] These and other objects and advantages of the present
invention will become more fully apparent from the following
detailed description taken with the accompanying drawings in
which:
[0052] FIG. 1 is a circuit diagram showing a cross current
compensation control system for a distribution system in accordance
with a first embodiment of the present invention;
[0053] FIG. 2 is a detailed circuit diagram showing the cross
current compensation control system for a distribution system in
accordance with the first embodiment of the present invention;
[0054] FIG. 3 is a detailed circuit diagram showing the cross
current compensation control system for a distribution system in
accordance with the first embodiment of the present invention;
[0055] FIG. 4 is a detailed circuit diagram showing a cross current
compensation control system for a distribution system in accordance
with a second embodiment of the present invention;
[0056] FIG. 5 is a detailed circuit diagram showing a cross current
compensation control system for a distribution system in accordance
with a third embodiment of the present invention;
[0057] FIG. 6 is a circuit diagram showing a cross current
compensation control system for a distribution system in accordance
with a fourth embodiment of the present invention;
[0058] FIGS. 7A to 7F are diagrams showing the operation of the
cross current compensation control system for a distribution system
in accordance with the fourth embodiment of the present
invention;
[0059] FIG. 8 is a circuit diagram showing a cross current
detection compensator of the cross current compensation control
system for a distribution system in accordance with the fourth
embodiment of the present invention;
[0060] FIG. 9 is a circuit diagram showing a cross current
detection compensator of a cross current compensation control
system for a distribution system in accordance with a fifth
embodiment of the present invention;
[0061] FIG. 10 is a circuit diagram showing a cross current
detection compensator of a cross current compensation control
system for a distribution system in accordance with a sixth
embodiment of the present invention;
[0062] FIG. 11 is a circuit diagram showing a cross current
detection compensator of a cross current compensation control
system for a distribution system in accordance with a seventh
embodiment of the present invention;
[0063] FIG. 12 is a circuit diagram showing the cross current
compensation control system for a distribution system in accordance
with the seventh embodiment of the present invention;
[0064] FIG. 13 is a circuit diagram showing a cross current
compensation control system for a distribution system in accordance
with an eighth embodiment of the present invention;
[0065] FIG. 14 is a circuit diagram showing the cross current
compensation control system for a distribution system in accordance
with the eighth embodiment of the present invention;
[0066] FIG. 15 is a circuit diagram showing a conventional
distribution system;
[0067] FIG. 16 is a circuit diagram showing a circuit for
explanation of the occurrence of a cross current in the
conventional distribution system;
[0068] FIG. 17 is a diagram showing a table for explanation of the
occurrence of a cross current in the conventional distribution
system;
[0069] FIG. 18 is a circuit diagram showing another conventional
distribution system;
[0070] FIG. 19 is a circuit diagram showing still another
conventional distribution system;
[0071] FIG. 20 is a circuit diagram showing yet still another
conventional distribution system; and
[0072] FIG. 21 is a circuit diagram showing yet still another
conventional distribution system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0073] Now, a description will be given in more detail of preferred
embodiments of the present invention with reference to the
accompanying drawings.
[0074] (First Embodiment)
[0075] A cross current compensation control system for a power
system in accordance with a first embodiment of the present
invention will be described with reference to the accompanying
drawings. FIG. 1 is a circuit diagram showing the cross current
compensation control system for a distribution system in accordance
with the first embodiment of the present invention. Also, the same
references in each figure denote the identical or corresponding
functions.
[0076] The first embodiment of the present invention is designed in
such a manner that a zero-phase circuit at the CT secondary side of
a distribution sending end is cross-connected so as to derive only
the cross current of a zero phase and the zero-phase current is
detected to compensate the cross current of the zero phase.
[0077] Referring to FIG. 1, reference numeral 40 denotes a cross
connection line, and 41 is a cross current detection compensator
such as a transformer. The same references as those in FIG. 15 have
identical functions and operations as those of the devices and
equipment of FIG. 15.
[0078] Also, FIG. 2 is a detailed circuit diagram showing the cross
current compensation control system for a distribution system in
accordance with the first embodiment of the present invention.
[0079] Referring to FIG. 2, reference 12-1 denotes an auxiliary CT
(current transformer) inserted into a zero-phase circuit at the
secondary side of a CT 12, and 22-1 is an auxiliary CT (current
transformer) inserted into a zero-phase circuit at the secondary
side of a CT 22. References 41-1 and 41-2 denote input terminals
(primary side terminals) of a cross current detection compensator
41, and 41-3 and 41-4 are output terminals (secondary side
terminals) of the cross current detection compensator 41. When the
input terminal 41-1 is +, the polarity of + appears in the output
terminal 41-3.
[0080] Also, in the drawing, reference 14-1, . . . 14-5, and 24-1,
. . . denote CTs (current transformers) disposed in switches 10-2,
. . . 30 and 20-2, . . . , and references 15-1, . . . 15-5 and
25-1, . . . denote section monitoring terminals that measure or
protect the section currents. In addition, references 50-1 and 50-2
denote zero-phase cross currents of a main circuit. References
61-1, 61-2, . . . 61-n, 62-1, 62-2, . . . 62-n are cross current
compensation lines of the zero phase to the respective section
monitoring terminals 15-1, . . . .
[0081] Subsequently, the operation of the cross current
compensation control system for a distribution system in accordance
with the first embodiment will be described with reference to the
accompanying drawings.
[0082] Referring to FIG. 2, when the switch 30 is closed to
generate a cross current 50, the cross currents 50-1 and 50-2 of
the zero-phase flow. Although a flowing direction represents the
clockwise direction, the counterclockwise direction may be applied
depending on the impedance and the condition of the system, but a
case of the clockwise will be described for convenience of
description.
[0083] On the other hand, a load current IL1-the zero-phase cross
current flows in the distribution line 10 side, a load current
IL2+the zero-phase cross current flows in the distribution 20 side,
and a current proportional thereto flows in the respective section
monitoring terminals. That is, the zero-phase cross current is
added to one current and the zero-phase cross current is subtracted
from the other current.
[0084] According to the first embodiment of the present invention,
the respective section monitoring terminals are compensated by
using a compensation current Ic outputted from the cross current
detection compensator 41. For example, the current component
corresponding to the above zero-phase cross current that appears on
the secondary side of the CT14-1 connected to the section
monitoring terminal 15-1 is canceled by the compensation current
Ic. The CT ratio (transformation ratio) of the CTs 12, 22 and the
auxiliary CTs 12-1, 22-1, CT14-1, . . . 14-5, and 24-1, . . . as
well as the compensation ratio (transformation ratio) of the cross
current detection compensator 41 are set in advance so that the
current component corresponding to the zero-phase cross current is
canceled by the compensation current Ic, that is, the current
components become at the same current level. Although a difference
between the load current at the distribution 10 side and the load
current at the distribution 20 side exists strictly, there arises
no problem because no load current appears in the zero-phase
circuit.
[0085] A case in which the switch 30 is opened will be described
with reference to FIG. 3. Although the load currents IL1 and IL2 of
the respective distributions 10 and 20 flow, since their load
current components do not flow in the cross current detection
compensator 41 connected to the cross connection line 40 of the
zero phase, there arises no problem.
[0086] In other words, the cross current compensation control
system for a distribution system in accordance with the first
embodiment of the present invention includes CTs 12 and 22 and
auxiliary CTs 12-1, 22-1 which are disposed at the respective
sending ends of the distributions 10 and 20 connected to the same
bus bar 6, a plurality of CTs 14-1, . . . 14-5 and 24-1, . . .
disposed in each of the sections of the distributions 10 and 20,
and input terminals 41-1, 41-2 and output terminals 41-3, 41-4, and
provides a cross current detection compensator 41 in which a cross
connection line 40 that cross-connects the secondary side of the
auxiliary CTs 12-1 and 22-1 is connected to the input terminal, and
cross current compensation lines 61-1, 61-2, . . . 61-n, 62-1,
62-2, . . . 62-n that connect the secondary sides of the plurality
of CTs 14-1, . . . 14-5 and 24-1, . . . in series are connected to
the output terminal. The cross current detection compensator 41
supplies a compensation current Ic to the cross current
compensation lines 61 and 62 so as to cancel a current component
corresponding to the cross current 50 that appears on the secondary
sides of the plurality of CTs 14 and 24 when detecting the cross
current 50 that circulates within the bus bar 6, the distributions
10, 20 and the distribution connection switch 30 that connects the
distribution 10 and the distribution 20.
[0087] As described above, according to the first embodiment of the
present invention, because the zero-phase input amounts of the
respective section monitoring terminals 15 and 25 are compensated,
the respective section monitoring terminals improve the reliability
of the protection function, in particular, the reliability with
respect to the ground protection. For this reason, the respective
section monitoring terminals can correctly grasp the state and
execute the sure protection.
[0088] (Second Embodiment)
[0089] A cross current compensation control system for a power
system in accordance with a second embodiment of the present
invention will be described with reference to the accompanying
drawings. FIG. 4 is a detailed circuit diagram showing the cross
current compensation control system for a distribution system in
accordance with the second embodiment of the present invention.
[0090] Referring to FIG. 4, reference 30-1 denotes a switch contact
that is controlled in association with the control of the switch
30, and 200 is a switch control unit that controls the switch and
the like and includes a calculator. Also, the same references as
those in FIGS. 1 and 2 except for the references 30-1 and 200 have
the identical functions.
[0091] In other words, when the switch control unit 200 opens the
switch 30, the switch contact 30-1 is closed. On the other hand,
when the switch control unit 200 closes the switch 30, the switch
contact 30-1 is opened. This control is effective in the case where
the distributions 10 and 20 operate independently.
[0092] In other words, the cross current compensation control
system for a distribution system according to the second embodiment
further includes, in addition to the elements of the above first
embodiment, the switch contact 30-1 that is connected between the
output terminals 41-3 and 41-4 of the cross current detection
compensator 41 and closed when the switch 30 that connects the
distributions 10 and 20 to each other is opened.
[0093] According to this embodiment, in the case where the
distributions 10 and 20 change from a loop system to a radial
system, the output side of the cross current detection compensator
41 that detects and compensates the cross current 50 caused by a
difference in the load amount or the like between the respective
distributions 10 and 20 are short-circuited, whereby the cross
current detection compensator 41 is not an obstacle in operation
without impeding the natural functions of the section monitoring
terminals 15 and 25.
[0094] (Third Embodiment)
[0095] Across current compensation control system for a power
system in accordance with a third embodiment of the present
invention will be described with reference to the accompanying
drawings. FIG. 5 is a detailed circuit diagram showing the cross
current compensation control system for a distribution system in
accordance with the third embodiment of the present invention.
[0096] Referring to FIG. 5, references 30-2 and 30-3 denote switch
contacts that are controlled in association with the control of the
switch 30. Also, the same references as those in FIG. 4 except for
the references 30-2 and 30-3 have the identical functions.
[0097] In other words, in the case where the switch 30 is opened by
the switch control unit 200, the switch contacts 30-1, 30-2 and
30-3 are closed. On the other hand, when the switch 30 is closed by
the switch control unit 200, those switch contacts 30-1, 30-2 and
30-3 are opened under control. Thus, the same function as that of
the above second embodiment or better is provided.
[0098] In other words, the cross current compensation control
system for a distribution system according to the third embodiment
further includes, in addition to the elements of the above first
embodiment, the switch contact 30-1 that is connected between the
output terminals 41-3 and 41-4 of the cross current detection
compensator 41 and closed when the switch 30 that connects the
distributions 10 and 20 to each other is opened, and the switch
contacts 30-2 and 30-3 that are connected between the input
terminals 41-1 and 41-2 of the cross current detection compensator
41 and closed when the switch 30 that connects the distributions 10
and 20 to each other is opened.
[0099] According to this embodiment, in the case where the
distributions 10 and 20 change from a loop system to a radial
system, the output side and the input side of the cross current
detection compensator 41 that detects and compensates the cross
current 50 caused by a difference in the load amount or the like
between the respective distributions 10 and 20 are short-circuited,
whereby the cross current detection compensator 41 is not an
obstacle in operation without impeding the natural functions of the
section monitoring terminals 15 and 25.
[0100] (Fourth Embodiment)
[0101] A cross current compensation control system for a power
system in accordance with a fourth embodiment of the present
invention will be described with reference to the accompanying
drawings. FIG. 6 is a detailed circuit diagram showing the cross
current compensation control system for a distribution system in
accordance with the fourth embodiment of the present invention.
FIG. 6 shows the cross current compensation control system in the
case where the power transformers operate in parallel.
[0102] Referring to FIG. 6, reference 8-1 denotes a PT
(transformer) connected to a bus bar 6, 8-2 is a PT (transformer)
connected to the bus bar 6-1, 9-1 is a voltage adjustment relay for
secondary voltage adjustment (voltage adjusting device) of the
power transformers (3, 4), and 9-2 is a voltage adjustment relay
for secondary voltage adjustment (voltage adjusting device) of the
power transformers (3-1, 4-1).
[0103] Also, in the figure, reference 70-1 denotes a secondary
current derivation CT (current transformer) of the power
transformers (3, 4), 70-2 is a secondary current derivation CT
(current transformer) of the power transformers (3-1, 4-1), 71 is a
cross connection line that cross-connects the secondary sides of
those CTs 70-1 and 70-2, 72 is a cross current detection
compensator, 73-1 is a cross current compensation output line, 73-2
is another cross current compensation output line, 74 is a cross
current measuring device for monitoring the cross current, and 90-1
and 90-2 are control lines. The same references as those in FIG. 18
except for the above references have the identical functions.
[0104] Subsequently, the operation of the cross current
compensation control system for a distribution system in accordance
with the fourth embodiment will be described with reference to the
accompanying drawings.
[0105] There has been well known a fact that, in FIG. 6, when the
bus bar connection circuit breaker 7 is closed, and a voltage
difference and an impedance difference exist between the power
transformers (3, 4) and the power transformers (3-1, 4-1), the
positive-phase and negative-phase cross currents 50 flow as shown
in the figure. The fourth embodiment of the present invention is
that the occurrence of the cross current is suppressed and
monitored by using the cross current 50.
[0106] According to the fourth embodiment, the input voltages to
the voltage adjustment relays 9-1 and 9-2 are controlled
(compensated) by the cross current 50, whereby the voltage
adjustment relays 9-1 and 9-2 raise the secondary voltage of the
power transformers at a lower voltage side and drop the secondary
voltage of the power transformers at a higher voltage side through
the control lines 90-1 and 90-2 under the control.
[0107] As to the zero-phase current in this case, in the case where
both of the primary side winding and the secondary side winding of
the power transformer are of the direct ground system, the
circulation of the zero-phase current exists within the closed
loop, but it is general that such a connection is not conducted in
the power transformer for a distribution system in order to prevent
the occurrence of a third higher harmonic wave.
[0108] FIGS. 7A to 7F are diagrams showing vectors of voltages and
currents at the respective points of the cross current compensation
control system in accordance with the fourth embodiment.
[0109] As the input current to the cross current detection
compensator 72 in this case, the three-phase current is employed as
it is. Referring to FIGS. 7A to 7F, FIG. 7A denotes voltages Ea, Eb
and Ec of the bus bars 6 and 6-1. FIG. 7B denotes a cross current
(circulating current) 50 at the power transformers (3-1, 4-1) side,
FIG. 7C is a cross current (circulating current) 50 at the power
transformers (3, 4) side, and a phase difference of even 180
degrees is defined between those cross currents 50. FIG. 7D is
cross current synthetic values of the respective phases, that is, a
phase A is indicated as Iatcc, a phase B is indicated as Ibtcc and
a phase C is indicated as Ictcc. Reference Vacc designates a
compensation voltage produced by using the cross current synthetic
value Iatcc of the phase A.
[0110] Also, in FIGS. 7A to 7F, FIG. 7E denotes an input voltage
received by the voltage adjustment relay 9-1 at the power
transformers (3, 4) side. Reference Ea is a voltage of the bus bar
6 obtained through the PT 8-1, -Vacc is a compensation voltage
supplied from the cross current detection compensator 72, and Ea"
is an input voltage received by the voltage adjustment relay 9-1.
In this case, because the voltage at the power transformers (3, 4)
side is low, when the cross current 50 circulates clockwise as
shown in the figure, the voltage adjustment relay 9-1 conducts the
control for raising the voltage through the control line 90-1 on
the basis of the input voltage Ea"=Ea-Vacc by adjusting the
secondary side tap of the power transformers (3, 4).
[0111] Further, in FIGS. 7A to 7F, FIG. 7F denotes an input voltage
received by the voltage adjustment relay 9-2 at the power
transformers (3-1, 4-1) side. Reference Ea is a voltage of the bus
bar 6-1 obtained through the PT 8-2, +Vacc is a compensation
voltage supplied from the cross current detection compensator 72,
and Ea' is an input voltage received by the voltage adjustment
relay 9-2. In this case, because the voltage at the power
transformers (3-1, 4-1) side is high, the voltage adjustment relay
9-2 conducts the control for dropping the voltage through the
control line 90-2 on the basis of the input voltage Ea'=Ea+Vacc by
adjusting the secondary side tap of the power transformers (3-1,
4-1).
[0112] A specific deriving method of the compensation voltage will
be described. FIG. 8 shows an example of a downsized transformer as
the cross current detection compensator 72.
[0113] In FIG. 8, when the detection current Ia of the cross
current 50 flows into a primary winding 72a, voltages develop
between both ends of resistors 72br, 72cr and 72dr connected to
secondary, third and fourth windings 72b, 72c and 72d,
respectively.
[0114] A voltage developed between both ends of the resistor 72br
of the secondary winding 72b is applied to the voltage adjustment
relay 9-1 through the cross current compensation output line 73-1
as the compensation voltage. Also, a voltage developed between both
ends of the resistor 72cr of the third winding 72c is applied to
the voltage adjustment relay 9-2 through the cross current
compensation output line 73-2 as the compensation voltage. Further,
a voltage developed between both ends of the resistor 72dr of the
fourth winding 72d is applied to the cross current measuring device
74 for the cross current amount monitor.
[0115] The resistors are inserted into the secondary, third and
fourth windings, respectively, but even if one resistor is inserted
into any one of the secondary, third and fourth windings, a
predetermined purpose of the present invention can be achieved,
that is, it is possible to derive the compensation voltage
proportional to the primary input current, and it is possible to
realize the initial purpose by using the compensation voltage for
the respectively intended purposes shown in the figure.
[0116] In other words, the cross current compensation control
system for a distribution system in accordance with the fourth
embodiment includes: the current transformers 70-1 and 70-2
disposed at the respective secondary sides of the power
transformers (3, 4) and (3-1, 4-1) connected to the same power
supply 1; the cross current detection compensator 72 connected to
the cross connection line 71 that cross-connects the secondary
sides of the current transformers 70-1 and 70-2; the voltage
adjustment relay 9-1 connected to the bus bar 6 connected to the
secondary sides of the cross current detection compensator 72 and
the power transformers (3, 4) through the PT8-1; and the voltage
adjustment relay 9-2 connected to the bus bar 6-1 connected to the
secondary sides of the cross current detection compensator 72 and
the power transformers (3-1, 4-1) through the PT8-2, in which: the
cross current detection compensator 72 outputs the compensation
voltage when detecting the cross current 50 that circulates within
the power transformers (3, 4) and (3-1, 4-1), the bus bars 6, 6-1,
and the switch 7 that connects the bus bars 6 and 6-1; the voltage
adjustment relay 9-1 controls the voltage adjustment tap at the
secondary side of the power transformers (3, 4) on the basis of the
voltage of the bus bar 6 and the compensation voltage; and the
voltage adjustment relay 9-2 controls the voltage adjustment tap at
the secondary side of the power transformers (3-1, 4-1) on the
basis of the voltage of the bus bar 6-1 and the compensation
voltage.
[0117] According to the fourth embodiment, it is possible to
minimize the cross current of the power transformers (3, 4) and
(3-1, 4-1) which are driven in parallel, and it is possible to
suppress the power loss caused by the useless cross current 50.
Also, it is possible to suppress the cross current from the power
transformers that are driven in parallel in the distribution
system, and it is possible to suppress the useless power loss as in
the above embodiment, thereby being capable of accurately
exhibiting the functions (measurement, protection, etc.) of the
respective section monitoring terminals disposed on the
distributions.
[0118] (Fifth Embodiment)
[0119] A cross current compensation control system for a power
system in accordance with a fifth embodiment of the present
invention will be described with reference to the accompanying
drawings. FIG. 9 is a detailed circuit diagram showing the cross
current compensation control system for a distribution system in
accordance with the fifth embodiment of the present invention.
[0120] Referring to FIG. 9, reference numeral 100 denotes a switch
contact that is controlled so as to be closed when the bus bar
connection circuit breaker 7 is opened. Reference numeral 200
denotes a switch control unit that controls switch unit and the
like.
[0121] Referring to FIG. 9, the bus bar connection circuit breaker
7 is opened by the switch control unit 200, the switch contact 100
is controlled so as to be closed. That is, in FIG. 6, in the case
where the respective power transformers (3, 4) and (3-1, 4-1) are
driven independently, because no cross current exists, it is
necessary to cancel the function of the cross current detection
compensator 72.
[0122] In other words, in the cross current compensation control
system for a power system in accordance with the fifth embodiment,
the cross current detection compensator 72 includes the switch
contact 100 that is connected between the input terminals of the
cross current detection compensator and closed when the bus bar
connection circuit breaker 7 that connects the bus bars 6 and 6-1
is opened.
[0123] According to the fifth embodiment, because the functions are
canceled at the input side of the cross current detection
compensator 72 when the parallel operation of the power
transformers (3, 4) and (3-1, 4-1) is canceled, the operation can
be conducted without any trouble of the system operation.
[0124] (Sixth Embodiment)
[0125] A cross current compensation control system for a power
system in accordance with a sixth embodiment of the present
invention will be described with reference to the accompanying
drawings. FIG. 10 is a circuit diagram showing the cross current
compensation control system for a distribution system in accordance
with the sixth embodiment of the present invention.
[0126] Referring to FIG. 10, reference numeral 101 and 102 denote
switch contacts that are controlled so as to be closed when the bus
bar connection circuit breaker 7 is opened by the switch control
unit 200. That is, in FIG. 6, in the case where the respective
power transformers (3, 4) and (3-1, 4-1) are driven independently,
because no cross current exists, it is necessary to cancel the
function of the cross current detection compensator 72. In the
sixth embodiment, because the cross current monitor output is
obtained, it is possible to monitor the load balancing between the
power transformers (3, 4) and (3-1, 4-1), and another function
increases.
[0127] In other words, in the cross current compensation control
system for a power system in accordance with the sixth embodiment,
the cross current detection compensator 72 includes the switch
contacts 101 and 102 that are connected between the output
terminals of the cross current detection compensator and closed
when the bus bar connection circuit breaker 7 that connects the bus
bars 6 and 6-1 is opened.
[0128] According to the sixth embodiment, because the function of
the cross current detection compensator 72 is canceled at the
secondary (output) side thereof, there can be provided a system
high in reliability in which the CT circuit is not opened since the
CT circuit is not directly controlled. Also, according to this
system, it is possible to monitor the balance state of the power
transformers (3, 4) and (3-1, 4-1).
[0129] (Seventh Embodiment)
[0130] A cross current compensation control system for a power
system in accordance with a seventh embodiment of the present
invention will be described with reference to the accompanying
drawings. FIG. 11 is a detailed circuit diagram showing the cross
current compensation control system for a distribution system in
accordance with the seventh embodiment of the present
invention.
[0131] Referring to FIG. 11, the respective switch contacts 100 to
102 have the same functions as those in the above-mentioned fifth
and sixth embodiments. That is, the seventh embodiment is directed
to the invention that combines the fifth and sixth embodiments
together. The switch contact 100 as well as the switch contacts 101
and 102 are closed by the switch control unit 200, thereby being
capable of more accurately canceling the cross current compensation
function.
[0132] In this example, it is possible to monitor the balancing of
the respective power transformers (3, 4) and (3-1, 4-1) in the
above fifth embodiment. However, in the seventh embodiment, the
monitor of the balancing can be realized by locating the
current/voltage converter having the same function, independently,
without any problem.
[0133] In this example, the respective power transformers (3, 4)
and (3-1, 4-1) are driven independently, and a case in which the
switch 30 is closed will be described.
[0134] In this case, as shown in FIG. 12, the seventh embodiment
synthesizes the above first, second or third embodiment with the
fourth, fifth, sixth or seventh embodiment without any problem in
actual operation.
[0135] In other words, in the cross current compensation control
system for a distribution system in accordance with the seventh
embodiment, the cross current detection compensator 72 includes the
switch contact 100 that is connected between the input terminals of
the cross current detection compensator and closed when the bus bar
connection circuit breaker 7 that connects the bus bars 6 and 6-1
is opened, and the switch contacts 101 and 102 that are connected
between the output terminals of the cross current detection
compensator and closed when the bus bar connection circuit breaker
7 that connects the bus bars 6 and 6-1 is opened.
[0136] According to the seventh embodiment, because the function of
the cross current detection compensator 72 is canceled at both of
the input side and the output side of the cross current detection
compensator 72, the cancellation of the function of the cross
current detection compensator 72 can provide the sure system.
[0137] (Eighth Embodiment)
[0138] Across current compensation control system for a power
system in accordance with an eighth embodiment of the present
invention will be described with reference to the accompanying
drawings. FIGS. 13 and 14 are diagrams showing in detail the cross
current compensation control system for a distribution system in
accordance with the eighth embodiment of the present invention.
[0139] Referring to FIG. 13, a power supply 1 and a power supply
1-1 represent different power supplies. In general, there are many
cases in which the power supply 1 and the power supply 1-1 are of
the same system and connected to each other through a bus bar of an
immediately upstream distribution substation.
[0140] Similarly, in this example, it is apparent that a cross
current similar to that in the above fourth embodiment occurs. The
cross current 50 in this case adds the phase difference of the
power supplies to the cross current in the fourth embodiment, and a
much larger cross current may flow depending on the system
operational state.
[0141] Referring to FIG. 13, in the case where the bus bar
connection circuit breaker 7 is closed, "voltage adjustment"
control of the respective power transformers (3, 4) and (3-1, 4-1)
is first conducted in the normal operation. If the "voltage
adjustment" is insufficient, it is apparent that the remarkable
cross current flows. The eighth embodiment is applied to the backup
of the "voltage adjustment". After the "voltage adjustment", the
above-mentioned fourth to seventh embodiments are applied in order
to surely minimize the cross current.
[0142] In addition, the cross current compensation in the case
where the bus bar connection circuit breaker 7 is opened and the
switch 30 is closed will be described.
[0143] That is, as shown in FIG. 14, in this case, similarly, the
input voltages to the voltage adjustment relays 9-1 and 9-2 are
made appropriate, thereby being capable of realizing the intended
function.
[0144] In other words, the cross current compensation control
system for a distribution system in accordance with the eighth
embodiment includes: the current transformer 70-1 disposed at the
secondary side of the power transformers (3, 4) connected to the
power supply 1; the current transformer 70-2 disposed at the
secondary side of the power transformers (3-1, 4-1) connected to
the power supply 1-1; the cross current detection compensator 72
connected to the cross connection line 71 that cross-connects the
secondary sides of the current transformers 70-1 and 70-2; the
voltage adjustment relay 9-1 connected to the bus bar 6 connected
to the secondary sides of the cross current detection compensator
72 and the power transformers (3, 4); and the voltage adjustment
relay 9-2 connected to the bus bar 6-1 connected to the secondary
sides of the cross current detection compensator 72 and the power
transformers (3-1, 4-1), in which: the cross current detection
compensator 72 outputs the compensation voltage when detecting the
cross current that circulates within the host bus bar (not shown)
that connects the power supplies 1 and 1-1, the power transformers
(3, 4) and (3-1, 4-1), the bus bars 6 and 6-1 and the bus bar
connection circuit breaker 7 that connects the bus bars 6 and 6-1;
the voltage adjustment relay 9-1 controls the voltage adjustment
tap at the secondary side of the power transformers (3, 4) on the
basis of the voltage of the bus bar 6 and the compensation voltage;
and the voltage adjustment relay 9-2 controls the voltage
adjustment tap at the secondary side of the power transformers
(3-1, 4-1) on the basis of the voltage of the bus bar 6-1 and the
compensation voltage.
[0145] According to the eighth embodiment, because there can be
provided the system that can minimize the cross current 50 in the
power transformers (3, 4) and (3-1, 4-1) of the different systems
which are driven in parallel and the distributions 10 and 20, it is
possible to reduce the useless power loss of the power transformers
(3, 4), (3-1, 4-1) and the distributions 10 and 20. Simultaneously,
the protection relays, the measuring devices and so on located at
the respective points of the distributions 10 and 20 can accurately
exhibit the natural functions thereof.
[0146] The foregoing description of the preferred embodiments of
the invention has been presented for purposes of illustration and
description. It is not intended to be exhaustive or to limit the
invention to the precise form disclosed, and modifications and
variations are possible in light of the above teachings or may be
acquired from practice of the invention. The embodiments were
chosen and described in order to explain the principles of the
invention and its practical application to enable one skilled in
the art to utilize the invention in various embodiments and with
various modifications as are suited to the particular use
contemplated. It is intended that the scope of the invention be
defined by the claims appended hereto, and their equivalents.
* * * * *